1. Field of the Invention
The present invention relates to a highly rigid propylene resin and to a blow-molded article made from the resin. More particularly, the invention relates to a highly rigid propylene resin which enables manufacture of large-sized blow-molded articles featuring light weight and excellent rigidity, dimensional stability, and heat resistance and which shows good drawdown resistance, as well as to a blow-molded article which is made from the resin and is advantageously used as, particularly, an automotive bumper, bumper beam, or a like part.
2. Description of the Related Art
Conventionally, propylene resins are used as general-purpose resins for manufacturing products of various shapes by various molding methods, including extrusion molding, injection molding, and blow molding. Because blow molding makes use of inexpensive molds and is a simple molding process attained through employment of integral molding, it has been intensively used for molding large-sized structural elements, mainly automobile parts. In view of specific gravity, rigidity, dimensional stability, and heat resistance, in many cases propylene resins are used as materials for such blow molding.
However, such conventionally used propylene resins are not necessarily satisfactory in terms of drawdown resistance or rigidity required for blow molding. Thus, various improvements on propylene resins have been developed. For example, there are disclosed a technique for improving drawdown resistance by manufacture of propylene resin through multi-step polymerization (Japanese Patent Publication (kokoku) No. 63-36609), a technique for improving rigidity by addition of a nucleating agent to propylene resin manufactured through multi-step polymerization (Japanese Patent Publication (kokoku) No. 3-74264), and a technique for improving drawdown resistance and resistance by addition of a nucleating agent to propylene resin manufactured through multi-step polymerization (Japanese Patent Application Laid-Open (kokai) 63-213547).
Although these techniques improve rigidity and drawdown resistance to a certain extent, in cases where large-sized blow-molded parts having a parison weight of not less than approximately 5 kg are molded, drawdown resistance attained by these techniques is insufficient, resulting in disabled molding or nonuniform thickness distribution in a blow-molded product. Therefore, the thus-obtained products tend to be unsatisfactory.
To solve the above problems, there has been developed, for example, a highly rigid propylene resin which is manufactured through multi-step polymerization, including polymerization of propylene and copolymerization of propylene and ethylene, and which allows manufacture of a large-sized blow-molded article featuring light weight and excellent rigidity, dimensional stability, and heat resistance, through adjustment of its melt index and the relation between its elongational viscosity and its melt index (USP5736613).
Although these techniques improve rigidity and drawdown resistance, in cases where blow molding is performed at a resin temperature of not higher than 230.degree. C., particularly not higher than 220.degree. C., blow molding may suffer an insufficient discharge rate, resulting in a reduction in productivity.
Accordingly, an object of the present invention is to develop a propylene resin having excellent rigidity and therefore showing good drawdown resistance, which enables manufacture of large-sized blow-molded articles featuring light weight and excellent rigidity, dimensional stability, and heat resistance, and being discharged by a sufficient amount in practice even during blow molding performed at a resin temperature of not higher than 220.degree. C., which enables excellent productivity.
Another object of the present invention is to provide a blow-molded article which is made from the propylene resin and is favorably used as, particularly, a large-sized automobile part, such as a bumper, a bumper beam, a seat back, an instrument panel, or a like part.